Cooling apparatus based on heat energy bound to working fluid in phase transition

ABSTRACT

The invention relates to a cooling apparatus ( 10 ) based on heat energy bound to working fluid in phase transition, the cooling apparatus comprising, for conducting elsewhere the heat energy generated by an electronic component ( 7 ), elongated cooling elements ( 1; 6   a,    6   b;    9   a - 9   f ) containing working fluid and extending in at least two different directions forming a matrix of cooling elements, and a heat conducting material ( 8; 15 ) binding the matrix of the cooling elements. The cooling elements ( 6   a,    6   b;    9   a - 9   f ) are arranged parallel to a circuit board level (A) and in at least one direction deviating from the circuit board level (A).

[0001] The invention relates to a cooling apparatus based on heat energybound to working fluid in phase transition, the cooling apparatuscomprising, for conducting elsewhere the heat energy generated by anelectronic component, elongated cooling elements containing workingfluid and extending in at least two different directions forming amatrix of cooling elements, and a heat conducting material binding thematrix of the cooling elements.

[0002] The need for cooling electronic components is an old and knownproblem, the significance of which is becoming more pronounced withincreasing power and integration densities, since all electroniccomponents generate heat, which has to be dissipated to accomplish anoptimal and reliable operation of the components. As the field ofelectronics continuously progress in a direction where increasingly highpowers are processed in increasingly small volumes, the dissipation ofheat from components has become a decisive planning criterion. Manycurrent electronic appliances need a cooling capacity that cannot beachieved by conventional metallic cooling fins.

[0003] New methods, such as heat pipes, have recently emerged along withconventional convection cooling. A heat pipe transfers heat veryefficiently; its effective thermal conductivity is of the order of 1000times that of copper. A heat pipe transfers heat from one end to theother as latent heat of phase transition, i.e. a liquid boils andevaporates in the hot end (an evaporator) of the pipe, and the createdpressure difference makes the vapour move to the other, cold end (acondenser) of the pipe, where the vapour emits its latent heat andreturns as a liquid to the evaporator, driven, for example, by capillaryforce. Heat pipes have been commercially available since 1960's.Basically they can respond to the heating needs of electronics, but theindustry has only recently begun to regard heat pipes as a reliable andadvantageous solution for the cooling problems of the most demandingelectronics applications.

[0004] Until now, heat pipes have generally transferred heat directlyfrom a hot component to the cooling apparatus by each component having aseparate heat pipe. It is extremely difficult, space-consuming andclumsy to place the heat pipes efficiently using this principal,particularly if there are several components requiring cooling on thesame circuit board. In prior art solutions the heat pipes are generallyeither at the circuit board level, in which case the condensers of thepipes are cooled by heat sinks or the like on the sides of the board, orvertically against the circuit boards, in which case the heat sink isparallel to the circuit board.

[0005] U.S. Pat. No. 5,527,588 presents a cooling apparatus for coolinga heat source using heat energy bound to working fluid in phasetransition. The cooling apparatus comprises a plurality of elongatedcooling elements extending parallel in at least two levels so as to forma two-dimensional matrix of cooling elements. In addition, the coolingapparatus comprises a heat conducting material binding the coolingelements. In this cooling apparatus the cooling elements are used totransfer heat in two directions, parallel to the level of the coolingelement matrix.

[0006] An object of the present invention is to provide a coolingapparatus for cooling electronic components which is superior to theones used until now and by which the problems of placing heat pipes canbe avoided.

[0007] The invention is based on multi-directional heat pipes integratedinto a metal matrix composite structure.

[0008] The cooling apparatus of the invention is characterized in thatthe elongated cooling elements are arranged parallel to the circuitboard level and in at least one direction deviating from the circuitboard level.

[0009] The preferred embodiments of the cooling apparatus of theinvention are the object of the dependent claims 2-5.

[0010] The invention can significantly improve the cooling of thermallycritical components and thus enable the implementation of more and moreefficient electronic systems and/or reduce the costs caused byconventional cooling systems.

[0011] In the following the invention will be described in greaterdetail by means of examples with reference to the accompanying drawings,in which

[0012]FIG. 1 shows a separate heat pipe and the structure thereof,

[0013]FIG. 2 schematically shows a cooling apparatus of an embodiment ofthe invention,

[0014]FIG. 3 shows the principal of a second embodiment of theinvention, and

[0015]FIG. 4 is the cross section of a cooling apparatus of a thirdembodiment of the invention.

[0016]FIG. 1 shows the structure and function of a heat pipe 1, knownper se, to be used in the invention. The heat pipe is shown partly incross section. The heat pipe, which is a gas tight tube-likeair-evacuated cooling element, has two ends. The object to be cooled, orin this case an end 2 that is near a heat generating electronic circuit,is called an evaporator and an opposite end 3 is called a condenser. Theheat pipe contains liquid working fluid which can be, for example,ammonia, water, acetone or methanol depending on the application. In theevaporator end 2 a liquid medium evaporates into gas and bindsevaporation heat, or latent heat, characteristic of the medium toitself. Evaporation causes a pressure gradient in the pipe forcing thevapour to flow along a middle part 4 of the pipe towards the condenser3. The vapour is adiabatically conveyed, whereby the variations inpressure and temperature are small. In the condenser the vapourcondenses back into liquid, conveying the evaporation heat to the heatsinks of the condenser. The working fluid is returned in liquid form tothe evaporator along a porous prime coat 5 forming the external threadof the heat pipe by capillary force. The basic function of a heat pipeis known in the prior art and will therefore not be explained here ingreater detail.

[0017] Alternatively micro heat pipes can preferably be used in thecooling apparatus of the invention in which case the capillary force iscaused by the geometry of the pipe. The basic function of a micro heatpipe is also known in the art and will not be described here in greaterdetail.

[0018]FIG. 2 shows an embodiment of the cooling apparatus based onworking fluid in phase transition of the invention. In accordance withthe invention the cooling apparatus is formed from a matrix of elongatedcooling elements i.e. heat pipes. The circuit board level is marked withthe letter A. In this example the heat pipes are two-dimensional; some 6a extending in parallel from a component 7 parallel to the circuit boardlevel A, and others 6 b vertically against the circuit board level.Corresponding main directions of the cooling effect are marked in theFigure. A heat conductive material 8, into which the heat pipes areembedded sees to it that heat emission from the component takes place atleast to some extent in three dimensions. The heat simultaneously movesas efficiently as possible from the component to the heat pipe throughthe heat conductive material 8.

[0019] Deviating from the case in FIG. 2, the vertical heat pipes 6 bcan alternatively be at another than vertical angle to the circuit boardlevel A. Alternatively the heat pipes 6 a may be arranged to extend invarious directions.

[0020] A suitable material for binding heat pipes is metal matrixcomposite preferably formed from silicon carbide (SiC) into which theheat pipes are placed and into which molten aluminium is brought forbinding the structure and for making the appearance compact, using, forexample, a conventional die casting method. The portion of siliconcarbide can be, for example, 70% and the portion of aluminium 30% byweight. Also other materials such as silicon nitride, aluminium oxideetc. can be used but their heat conductivity is inferior to that ofsilicon carbide.

[0021] The circuit board may comprise one or several componentsrequiring cooling. When the heat pipes have been integrated into acomposite the cooling apparatus according to the invention becomes afixed structure which is easy to mount, space-saving and durable. Theinvention can be utilized particularly in the cooling of powerelectronics, e.g. processors, ASIC circuits, power sources and thecomponents of RF electronics.

[0022] According to the principal shown in FIG. 3 the heat pipes 9 a-9 fcan also be arranged to form a three dimensional network around thecomponent 7 to be cooled, in which case lost heat can be evenly dividedinto a cooling surface area as wide as possible. Thus, the coolingeffect takes place mainly in all five directions shown by the heat pipes9 a-9 f presented in rough outline in the Figure or in their desiredsubset according to the space and need of the heat pipes. The circuitboard level is marked with the letter A.

[0023] In the case shown in FIG. 3 the cooling effect can be extended totake place in six directions if two cooling apparatuses according toFIG. 3 are used and the component to be cooled is placed between thetwo.

[0024] As a third alternative the location of the heat pipes at thecircuit board level vertically against each other so as the heatemission from the component takes place parallel to the circuit boardlevel in two dimensions can be mentioned.

[0025] In the embodiment in FIG. 4 cavities 10 a-10 c corresponding withthe topology of a circuit board 12 and components 13 a, 13 b, 13 ctherein are worked into the cooling apparatus 10. The cooling apparatus10 is then preferably of the same size as the circuit board 12. Then theheat pipes 11 can be accurately imposed in the immediate vicinity of thehot components and the number and direction of the heat pipes can bevaried according to what is most appropriate. By using very smallso-called micro heat pipes, the heat pipes can be placed at desiredlocations of the cooling apparatus as areas of desired sizes, with noneed to plan how to place individual pipes.

[0026] The heat pipes can be built to either transfer heat to externalcooling fins 16 or only to balance the temperature distribution of thecircuit board metal matrix combination 12, 15 by conveying heat from thehot components 13 a, 13 b, 13 c to the other material. Cooling theactual circuit board 12 is in some cases also preferable since up to 70%of the lost heat of the components can vanish into it. In this case thecircuit board can be used as a heat sink and its temperaturedistribution becomes homogeneous.

[0027] Such a worked structure can be formed to be very compactparticularly if there are several components to be cooled on the circuitboard in which case the cooling apparatus is easy to handle and installand the circuits are also easy to maintain. A thermal grease 14 oranother medium is preferably placed between the components and thecooling apparatus to improve thermal contact between the components andthe cooling apparatus.

[0028] It is obvious to one skilled in the art that the differentembodiments of the invention are not restricted to the examplesdescribed above but can vary within the scope of the claims below.

1. A cooling apparatus (10) based on heat energy bound to working fluidin phase transition, the cooling apparatus comprising, for conductingelsewhere the heat energy generated by an electronic component (7),elongated cooling elements (1; 6 a, 6 b; 9 a-9 f) containing workingfluid and extending in at least two different directions forming amatrix of cooling elements, and a heat conducting material (8; 15)binding the matrix of the cooling elements, characterized in that theelongated cooling elements (6 a, 6 b; 9 a-9 f are arranged parallel to acircuit board level (A) and in at least one direction deviating from thecircuit board level (A).
 2. A cooling apparatus as claimed in claim 1 ,characterized in that the elongated cooling elements (9 a-9 f) arearranged vertically against each other parallel to the circuit boardlevel (A) and vertically against the circuit board level (A).
 3. Acooling apparatus as claimed in claim 1 , characterized in that theelongated cooling elements (6 a, 6 b) are arranged in parallel parallelto the circuit board level (A) and vertically against the circuit boardlevel (A).
 4. A cooling apparatus as claimed in any one of claims 2-3,characterized in that a cavity (10 a-10 c) according to the form of atleast one component (13 a-13 c) to be cooled, located on the circuitboard, is formed on the surface, which is against the circuit board (12)of the cooling apparatus (10).
 5. A cooling apparatus as claimed inclaim 4 , characterized in that a medium (14) improving the thermalcontact between the components (13 a-13 c) to be cooled and the coolingapparatus (10) is placed between the two.